This invention relates to storage-stable, one-package, self-crosslinking aqueous latices useful as adhesives, coatings or binders.
Vinyl ester based and ethylene-vinyl acetate based polymers have been used for some time in adhesives, coatings, and binders. When the polymers contain a functional group such as a carboxyl group, the polymers are cured by the addition of a co-reactant (i.e., crosslinker) to the polymer emulsion or solution. When the polymers contain a functionality that is self-reactive, the use of a co-reactant species per se is not necessary. The advantages of self-crosslinking polymer systems are their simplicity, economy, and efficiency.
Latices containing self-crosslinking polymers prepared from cationic functional monomers are disclosed in U.S. Pat. Nos. 3,678,098, 3,694,393 and 3,702,799 issued July 18, Sept. 26, and Nov. 14, 1972 to Sheldon N. Lewis et al. and U.S. Pat. Nos. 3,095,390 and 3,287,305 issued June 25, 1963 and Nov. 22, 1966 to A. Maeder. Latices containing self-crosslinking polymers prepared from the non-ionic functional monomers 3-chloro-2-hydroxypropyl methacrylate are described in the technical data sheet of Alcolac for Sipomer.RTM..CHPM. 3-Chloro-2-hydroxypropyl acrylate (CHPA) is disclosed as useful in various polymers or polymeric formulations such as a polymer of butyl acrylate, methyl methacrylate, acrylonitrile, acrylic acid and CHPA useful in a printing paste (see Ger. Pat. No. 1,282,600 issued 11/14/68 to K. Craemer et al., CA 70 48583q); polymers containing CHPA and at least one functional monomer from a chemically different class which are useful as curing agents for gelatin layers (see Ger. 1,109,875 issued June 29, 1961 to E. J. Birr et al., CA 56 1094b); adhesives from a proteinaceous matter, vinyl acetate, and CHPA (see U.S. Pat. No. 3,314,905 issued Apr. 18, 1967 to S. B. Luce et al.); a polymer of methyl methacrylate, butyl acrylate, and CHPA in an aqueous dispersion which is useful for coating plasticized poly(vinyl chloride) (see Ger. Pat. No. 1,231,372 issued Dec. 29, 1966 to G. Welzel et al., CA 66 56674c); polymers of styrene, 2-ethylhexyl acrylate, N-(butoxymethyl) methacrylamide, acrylic acid, and optionally CHPA useful in printing inks as a self-crosslinkable binder (see Fr. Pat. No. 1,573,457 issued July 4, 1969 to Badische Anilin- und Soda-Fabrik A.G., C.A. 42 56820 w); a polymer of butyl acrylate, acrylonitrile, and CHPA in a cotton fabric dye composition (see Fr. Pat. No. 1,513,899 issued Feb. 16, 1968 to Badische Anilin- und Soda-Fabrik A.G., CA 70 97911d); a crosslinkable ethylacrlyate-CHPA copolymer useful in admixture with a solution or dispersion of another crosslinking monomer as an adhesive coating (see Ger. Offen. No. 1,904,743 published Sept. 10, 1970 by H. Reinhard et al., CA 73 99733h); and a polymer of 2-ethylhexyl acrylate, n- and tert-butyl acrylate, N-vinyl pyrrolidinone, acrylic acid, and CHPA useful as an aqueous adhesive coating (see Ger. Offen No. 1,911,306 published Sept. 10, 1970 by H. Reinhard, CA 73 110607t). The polymers are crosslinked under alkaline conditions with or without the use of heat. Alkali (e.g., sodium and ammonium hydroxide) and alkaline salts (e.g., sodium sesquicarbonate and sodium or potassium carbonate) are used for the cure.
The major problem associated with these alkaline-curable latices is their short pot life. The need to blend the curing agent with the latices just prior to their use is inconvenient. There is a need therefore for a one-package latex which is shelf-stable but still capable of self-crosslinking after application.
SUMMARY OF THE INVENTION
The present invention provides a shelf-stable, curable, one-package latex which consists essentially of a dispersion of an alkaline-curable, self-crosslinking emulsion polymer in water and a salt of an organic acid having the formula ##STR1## where R is H or a C.sub.1 -C.sub.6 straight or branched chain alkyl or C.sub.2 -C.sub.6 alkenyl, M is an alkali, alkaline earth, or heavy metal and n is the valence number of M; the self-crosslinking polymer comprising a vinyl polymerizable monomer and a polymerizable cationic or non-ionic self-crosslinking monomer containing halohydrin groups; the salt being present in an amount effective to cure the polymer, with or without the application of heat, after application of the dispersion, and removal of the water therefrom. Suitable self-crosslinking monomers include cationic quaternary ammonium monomers having the formula ##STR2## and nonionic monomers having the formula ##STR3## where R.sup.1 and R.sup.5 are hydrogen or a methyl group; A is --O-- or ##STR4## with R.sup.4 being hydrogen or a C.sub.1 -C.sub.3 alkyl group; R.sup.2 and R.sup.3 are independently C.sub.1 -C.sub.6 alkyl groups; X is a halogen; Y is an anion; and n is 1-4. Typically the self-crosslinking polymer comprises about 90-99.9% by weight of the vinyl polymerizable monomer and about 0.1-10% by weight of the halohydrin-containing monomer. As used herein, the term "stable" applies to a latex that is shelf-stable (i.e., where there is no increase in grits and/or viscosity and/or phase separation in the latex and/or coagulation of the polymer) and is still capable of curing (i.e. crosslinking) to provide the desired end use performance.
The salt may be introduced into the latex by addition to an already prepared polymer latex or by inclusion in one of the charges used in the preparation of the polymer latex. Typically a substantially equivalent amount of salt, based on the moles of cationic or non-ionic functional monomer, will be effective. The effect of the salt on the polymer may be determined by studying the % insolubles formed after cure. With acrylate-based polymers the use of additional salt will lead to a reduction in % insolubles. With vinyl acetate-based polymers, however, salt amounts in excess of that needed to provide the equivalent amount of alkali do not reduce the % insolubles. Hence, the effective amount of salt to be used may vary with the salt, the type and amount of functional monomer used, and the polymer type (acrylate vs. vinyl acetate). It will also depend upon the degree of crosslinking desired, which may vary depending upon the intended end use.
Typical non-functional vinyl polymerizable monomers include vinyl esters, alkyl (meth)acrylates, styrene and mixtures thereof as well as ethylene-vinyl acetate and mixtures thereof with other vinyl polymerizable monomers. The monomer type and amount depends upon the end use (i.e., laminating adhesive, pressure sensitive adhesive, binder, and the like).
Typical functional monomers include the cationic monoethylenically unsaturated ester monomers derived from N,N-dialkylaminoalkyl esters of (meth)acrylic acid and epihalohydrins, the cationic monoethylenically unsaturated ester monomers derived from N,N-dialkylaminoalkyl amides of (meth)acrylic acid and epihalohydrins, and the non-ionic ethylenically unsaturated ester monomers derived from (meth)acrylic acid and epihalohydrins. It may also be possible to use other monomeric polymerizable chlorohydrin esters or amides prepared by the reaction of epichlorohydrin and other unsaturated acids such as crotonic, furmaric, maleic, and itaconic acids or their N,N-dialkyl-aminoalkyl esters or amides.
In the vinyl ester polymers, the vinyl ester is present in amounts of at least about 5%, the functional monomer is present in amounts of about 0.1-10%, and any optional vinyl polymerizable monomer is present in amounts of about 0-94.9%. In the ethylene-vinyl acetate polymers, the monomer amounts are about 0.5-40% ethylene, up to about 89.5% vinyl acetate, about 0.1-10% cationic monomer, and 0-5% of any optional vinyl polymerizable monomer. The amounts are by weight and total 100%. As used herein, the term "functional" monomers refers to those monomers containing halohydrin or epoxide groups which are capable of self-crosslinking under alkaline conditions.
Suitable salts include alkali, alkaline earth, or heavy metal salts of organic acids (e.g. sodium, potassium, calcium, aluminum, or lead formate, acetate, propionate and the like). The salt acts as a latent curing agent which, after application of the latex and the removal of the water, cures the self-crosslinking monomer. Since the salt does not raise the pH of the latex during storage, the shelf-stability is good. It is believed that during drying the pH is raised by the in situ generation of a strong alkali when the organic acid anion is removed as a volatile acid. Drying may be carried out at room temperature or an elevated temperature depending upon the end use and, more importantly, the volatility of the acid. For example, sodium acetate is capable of curing the polymers at room temperature. The volatile acetic acid is removed with the water during drying and the sodium hydroxide produced is believed to effect the cure.